High Bandwidth Atomic Magnetometery with Continuous Quantum Nondemolition Measurements

We describe an experimental study of spin-projection noise in a high sensitivity alkali-metal magnetometer. We demonstrate a fourfold improvement in the measurement bandwidth of the magnetometer using continuous quantum nondemolition measurements. Operating in the scalar mode with a measurement volume of $2{\mathrm{cm}}^3$ we achieve magnetic field sensitivity of $22\mathrm{fT}/{\mathrm{Hz}}^{1/2}$ and a bandwidth of 1.9 kHz with a spin polarization of only 1%. Our experimental arrangement is naturally backaction evading and can be used to realize sub-fT sensitivity with a highly polarized spin-squeezed atomic vapor.

Figure 1

Apparatus for high-bandwidth QND scalar magnetometry.

Figure 2

Measured spin noise power spectrum of unpolarized atoms (solid line) with a fit to a Lorentzian plus flat photon shot noise background (dashed line). The averaging time for the noise spectrum is 350 s.

Figure 3

Lock-in output amplitude noise spectrum for polarized atoms (solid line) and unpolarized atoms (dashed line). The signal expected in the magnetometer for a $22{\mathrm{fT}}_{\mathrm{rms}}$ magnetic field is shown with dots. The spikes in the noise spectrum for polarized atoms are from technical magnetic noise.

Figure 4

Experimentally observed magnetic field noise spectral density corrected for the frequency response of the magnetometer (solid line). The spikes are due to narrow-band magnetic noise. The dashed line shows expected magnetometer sensitivity for a demolition measurement assuming a flat noise spectrum. The shaded area represents improvement in the magnetometer sensitivity at high frequencies as a result of QND measurements. For QND measurements the sensitivity decreases by $\sqrt{2}$ at 1.9 kHz.